Adjacent protons can act as magnets and affect the amount of radiation required for proton resonance in NMR. This interaction causes blank. Spin-spin coupling, resulting in peak splitting; rapid relaxation and unreliable integration results; increased shielding, forcing the chemical shift to be lower; decreased shielding, forcing the chemical shift to be higher.
This question is specifically about the protons in proton NMR. Proton NMR is a really versatile technique that can give us a lot of information, particularly about organic compounds. Think about looking at an NMR spectrum. What are the features that you really look for?
One important aspect of proton NMR is that the integration of the peaks is quantitative. What this means is that the area underneath the peaks is directly proportional to how many protons there are in that environment. Let’s have a look at an example.
Let’s have a look at the proton NMR spectrum of acetic acid. In this molecule, we have two different proton environments. For the proton attached to the oxygen, we can see that it forms a peak, a small peak, at about 11 ppm. The methyl protons have a peak at just above two. And this one is significantly taller.
If you want to measure the heights of these peaks on a real spectrum, you’d find that they were in a one-to-three ratio. This is because there’s one hydrogen in the first environment and three in the other environment. Integration in a proton NMR spectrum is hugely useful for finding out numbers of protons in each environment. And it’s very reliable. This means that we could already rule out one of the answers given. The second answer of rapid relaxation and unreliable integration results cannot be correct. So what other information can we get from a proton NMR spectrum?
Chemical shift is perhaps the next important feature of a spectrum. Chemical shift can tell us what type of environment a proton is in and what sort of atoms are nearby. Chemical shift is closely related to the electronegativity of substituents in a molecule. We can draw up a guide to show roughly where we would expect each type of proton to appear.
Between approximately naught and three ppm, we would expect to find peaks relating to saturated environments, things like a methyl, a CH₂, a CH, as long as they’re not next to an oxygen. If these groups are next to an oxygen atom, the peak that they produce will be shifted downfield to higher chemical shift. This is because the oxygen is an electronegative element. This has an electron-withdrawing effect.
As the oxygen atom withdraws electron density from the proton environment in question, we say that the proton has less shielding or is deshielded. By deshielding the proton environment, you make it more susceptible to the magnetic field in the NMR spectrometer, which forces the chemical shift higher.
We can fill in the rest of the guide as follows. Alkenes appearing approximately 4.5 to 6.5 ppm, protons and unsaturated carbons like benzene and aromatics between six and half and eight and half ppm, and finally protons attached to unsaturated carbons, which are also next to an oxygen. But how does this relate to our question?
The bottom two answers for this question are increased shielding, forcing the chemical shift to be lower, or decreased shielding, forcing the chemical shift to be higher. We’ve already mentioned the last option: the decreased shielding, forcing the chemical shift to be higher. And we’ve attributed that to potentially nearby electronegative atoms or protons which are attached to unsaturated carbons.
This question is asking about adjacent protons. Since protons are not electronegative atoms, this final answer cannot be the interaction caused by adjacent protons. So we can rule it out. Likewise, for increased shielding, forcing the chemical shift to be lower, we’re looking at the opposite, so something that is potentially electron-donating.
Adjacent protons wouldn’t significantly increase shielding and therefore wouldn’t affect the chemical shift to make it much lower. So this answer is also incorrect. This brings us to the top answer and looking at peak-splitting patterns in NMR spectroscopy.
To explain this, let’s look at another example. Let’s consider the proton NMR spectrum of cytosine. Cytosine produces a spectrum a bit like this. Note that I’ve not included the entire spectrum. This shows the two peaks caused by the protons attached to the heterocyclic ring. I’ve labelled them HX and HA. Notice that each peak is split into two smaller peaks. This pattern is called a doublet. But why do we get two small peaks for each proton?
This is where we look at spin-spin coupling. If there was no interaction between HX and HA, this is the type of spectrum you would see: two single lines, singlets. But this is not the case. Let’s consider the effect of HX upon HA. There are two possible interactions.
If HX has a spin which is aligned with the applied field, then HA will experience both the applied field and the field of HX. This will drag the peak downfield slightly. If HX is aligned against the applied field, then HA is going to feel the applied field minus that of HX. And this will shift the peak upfield.
In reality, what we see is a mixture of these two. So we get a doublet. Remember of course that we’ve only looked at the effect of HX on HA. But the reverse is also true. HA has the same effect on HX. So HX is also a doublet. This explains why adjacent protons would cause spin-spin coupling, which results in peak splitting. So the answer to our question is the first option: spin-spin coupling, resulting in peak splitting.